JPH06287063A - Silicon nitride ceramic - Google Patents

Abstract

PURPOSE:To provide a silicon nitride ceramic having highly reproducible high- temperature strength and high-temperature creep characteristics, etc., irrespective of silicon nitride stock therefor. CONSTITUTION:The silicon nitride ceramic predominant in silicon nitride, consisting of a sintered compact containing an appropriate amount of a sintering auxiliary component. Besides, this sintered compact has the following characteristics: (1) containing a total of <=300ppm of fluorine and chlorine (independently, <=150ppm of fluorine and <=250ppm of chlorine) and (2) the grain boundary phase therein has been crystallized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to silicon nitride ceramics excellent in high temperature characteristics.

[0002]

2. Description of the Related Art As a ceramic-based structural material,
Conventionally, mainly silicon nitride ceramics, silicon carbide ceramics, sialon ceramics containing Si-Al-ON as a main constituent element have been used. Among them, silicon nitride ceramics are superior in room temperature strength to silicon carbide ceramics and sialon ceramics, and have a high fracture toughness value, and have various characteristics such as automobile parts members and gas turbine blades. Has been attempted to be applied to a high strength heat resistant structural material.

By the way, the silicon nitride ceramics have various advantages as described above, but have a drawback that the high temperature characteristics such as high temperature strength and high temperature creep characteristics are inferior to those of sialon ceramics. Under such circumstances, various attempts have conventionally been made to improve the high temperature characteristics of silicon nitride ceramics.

For example, as a method for improving the high temperature characteristics of silicon nitride ceramics, a method of crystallizing a grain boundary phase mainly composed of sintering additive components is known (Suzuki, Furukawa, FC Report 3 [ 4] 15 (1985)). Since silicon nitride is difficult to sinter, liquid phase sintering with additives is mainly applied to the sintering. As additives that act as sintering aids for silicon nitride, oxides of rare earth oxides, aluminum oxide, aluminum nitride, hafnium, tantalum, niobium, etc. are known, but these are amorphous grain boundaries after sintering. If it remains as a phase, it becomes a factor for lowering the strength at high temperatures, so attempts have been made to crystallize the grain boundary phase. Further, it has been attempted to improve the high temperature characteristics by the composition of the sintering aid itself.
No. 62-153169) is known.

[0005]

As described above, various attempts have heretofore been made to improve the high temperature characteristics of silicon nitride ceramics. However, when a silicon nitride powder produced by an inexpensive process such as a direct nitriding method of metallic silicon is used, there is a problem that even if an attempt is made to improve the high temperature characteristics as described above, the effect is difficult to obtain. At present, the cause has not been clarified. Therefore, it is strongly desired to develop a technique capable of improving the high temperature characteristics of silicon nitride ceramics, such as high temperature strength and high temperature creep characteristics, regardless of the type of raw material powder of silicon nitride.

[0006] The present invention has been made to address such a problem, and it is
It is an object of the present invention to provide a silicon nitride ceramics capable of obtaining excellent high temperature strength and high temperature creep characteristics with good reproducibility.

[0007]

Means and Actions for Solving the Problems In order to achieve the above-mentioned object, the present inventors have conducted various studies on the characteristic deterioration factors of silicon nitride ceramics at high temperatures, and as a result, they are contained in the sintered body. It was found that halogen elements such as fluorine and chlorine hinder the crystallization of the grain boundary phase and deteriorate the high temperature characteristics.

The present invention was made based on the above findings, and the silicon nitride ceramics of the present invention is a silicon nitride composed of a sintered body containing silicon nitride as a main component and an appropriate amount of a sintering additive component. Ceramics, the total amount of fluorine and chlorine contained in the sintered body is 300ppm or less, and the grain boundary phase present in the crystal grain boundary of silicon nitride in the sintered body is crystallized Is characterized by.

The silicon nitride ceramics of the present invention comprises a sintered body containing silicon nitride as a main component and an appropriate amount of a sintering aid component. The total amount of fluorine and chlorine contained in the sintered body is set to 300 ppm or less. Fluorine and chlorine inhibit crystallization of the grain boundary phase in the sintered body, and when the total amount of these halogen elements exceeds 300 ppm, the proportion of the amorphous phase in the grain boundary phase increases, and Even if the boundary phase is crystallized, a compound having a low melting point is likely to be generated, so that the high temperature strength and the high temperature creep property are deteriorated.

That is, by setting the total amount of fluorine and chlorine in the sintered body to 300 ppm or less, the grain boundary phase can be stably crystallized and the crystalline compound constituting the grain boundary phase is increased. The compound can be a melting point compound, and these can improve the high temperature strength and the high temperature creep characteristics with good reproducibility. Of the above halogen elements, fluorine has a large effect on high-temperature characteristics, so its content is 15
It is preferably 0 ppm or less. Also, the chlorine content is
It is preferably 250 ppm or less. However, the total amount of these is within the above range. A more preferred range of the total content of fluorine and chlorine is 200 ppm or less, a more preferred content of fluorine is 100 ppm or less, and a more preferred content of chlorine is
It is below 200ppm.

As described above, the silicon nitride ceramics of the present invention crystallize the grain boundary phase existing in the crystal grain boundaries of silicon nitride by defining the contents of fluorine and chlorine in the sintered body. It is preferable that the grain boundary crystal phase is mainly composed of a melilite phase. Various compounds are known as the compounds constituting the grain boundary crystal phase of silicon nitride, and particularly the melilite phase (Si ₃ N ₄ and Y ₂ O
₃ and of 1: 1 compound) is known as a compound having a high melting point. As described above, by mainly constituting the grain boundary crystal phase with the melilite phase, it becomes possible to further improve the high temperature characteristics.

As the sintering aid component used in the present invention, various compounds generally used as a sintering aid for silicon nitride, that is, various compounds functioning as a sintering accelerator for silicon nitride can be used. Although possible, it is particularly preferable to use the rare earth oxide-hafnium oxide-aluminum nitride system. Examples of the rare earth oxide include yttrium oxide, ytterbium oxide, and the like, but yttrium oxide is preferable from the viewpoint of densification during sintering, manufacturing cost, and the like. The rare earth oxide-hafnium oxide-aluminum nitride-based sintering aid as described above is an aid system that facilitates the formation of the melilite phase described above, and thus contributes to the improvement of high-temperature characteristics. The total amount of such sintering aids added is preferably in the range of 6 to 15% by weight of the total composition. If the addition amount of the sintering aid is less than 6% by weight, the sintered body cannot be sufficiently densified, and the addition amount is 15% by weight.
When it exceeds, the ratio of the mother phase decreases relatively,
It may impair the original properties of silicon nitride.

The silicon nitride ceramics of the present invention is manufactured, for example, as follows. That is, first, an appropriate amount of sintering aid powder is added to silicon nitride powder, mixed sufficiently, and then molded into a desired shape. As the silicon nitride powder, various powders such as a powder by a direct nitriding method of metal Si and a powder by an imide decomposition method can be used. Then, so that the amount of fluorine and chlorine after sintering is in the above range,
Depending on the contents of fluorine and chlorine in the silicon nitride powder and the sintering aid powder, defluorination treatment or dechlorination treatment is performed at the powder stage or after forming a molded body. Examples of the dehalogenation treatment include heat treatment in an ammonia-containing atmosphere.

After this, the above-mentioned molded body is placed in an inert atmosphere,
By firing at a temperature of about 1600 ° C to 1900 ° C, the silicon nitride ceramics of the present invention can be obtained. As the sintering method, various methods such as an atmospheric pressure sintering method, an atmosphere pressure sintering method, a hot pressing method, a hot isostatic pressing method (HIP), or a combination thereof can be adopted. It is possible.

[0015]

EXAMPLES The present invention will be described below with reference to examples.

Examples 1 to 5 and Comparative Examples 1 to 3 First, Si _{3 having} an average particle size of 0.6 μm was formed by the metal Si direct nitriding method.
N ₄ powder was prepared. The amount of fluorine contained in this Si ₃ N ₄ powder was 600 ppm, and the amount of chlorine was 400 ppm. For such Si ₃ N ₄ powder, Y ₂ O with an average particle size of 1 μm
4% by weight of ₃ powder, 7% by weight of AlN powder with an average particle size of 1 μm
%, And 1% by weight of HfO ₂ powder having an average particle size of 1 μm were mixed and thoroughly mixed with a ball mill to prepare a raw material powder. Next, an appropriate amount of a binder was added to this raw material powder, and the mixture was further thoroughly mixed and dried, and then a 50 mm × 50 mm × 5 mmt plate-shaped molded body was produced by press molding.

A plurality of such compacts were prepared and subjected to heat treatment in a NH ₃ stream as defluorination / chlorine treatment. By changing the conditions of this defluorination / chlorination treatment, molded articles having various fluorine and chlorine contents were obtained.

For comparison with the present invention, a molded article not subjected to defluorination / chlorine treatment and a molded article whose treatment conditions are relaxed so that the fluorine and chlorine contents fall outside the range of the present invention are the same. It was made.

Each of the compacts according to the above-mentioned Examples and Comparative Examples was subjected to a degreasing treatment in a nitrogen gas atmosphere, and then subjected to atmospheric pressure sintering in a nitrogen gas atmosphere at 1750 ° C. for 4 hours. Silicon nitride ceramics were obtained respectively.

The fluorine and chlorine contents in each silicon nitride ceramic thus obtained were measured by chemical analysis. Moreover, the degree of crystallization of the grain boundary phase and whether or not the melilite phase was present in the grain boundary phase were confirmed by X-ray diffraction. Furthermore, 3 x 4 from each of the above silicon nitride ceramics
A × 40 mm test piece was cut out and the 3-point bending strength was measured at 1250 ° C. These results are summarized in Table 1.

[0021]

[Table 1] As is clear from Table 1, the total amount of fluorine and chlorine
300ppm or less, fluorine amount 150ppm or less, and chlorine amount 25
The silicon nitride ceramics according to the respective examples, which have a content of 0 ppm or less, have crystallized grain boundary phases and have a melilite phase as a main constituent phase thereof, whereby excellent high temperature strength is obtained. The three-point bending strength of the above silicon nitride ceramics at room temperature was 8
It is in the range of 00 to 950 MPa, and it can be seen that the deterioration of strength at high temperatures is suppressed. Further, the high temperature creep properties of these silicon nitride ceramics according to Example 1 and Comparative Example 2 were evaluated. As an evaluation, a tensile creep test was performed at a stress of 250 MPa in an atmosphere of 1250 ° C. As a result, Example 1 had a life of 10 ⁴ hours, and Comparative Example 2 broke at 10 ² hours. It has been confirmed that the silicon nitride ceramics according to the present invention also have excellent high temperature creep properties.

[0022]

As described above, according to the silicon nitride ceramics of the present invention, high temperature characteristics such as high temperature strength and high temperature creep characteristics can be improved with good reproducibility regardless of the type of silicon nitride raw material. Becomes Therefore, it is possible to provide silicon nitride ceramics suitable as a structural material used under various high temperature atmospheres.

[0023]

Claims (3)

[Claims] 1. A silicon nitride ceramic comprising a sintered body containing silicon nitride as a main component and an appropriate amount of a sintering aid component, wherein the total amount of fluorine and chlorine contained in the sintered body is 300 ppm.
The silicon nitride ceramics is characterized in that the grain boundary phase existing below and present in the crystal grain boundary of silicon nitride in the sintered body is crystallized. 2. The silicon nitride ceramics according to claim 1, wherein the amount of fluorine contained in the sintered body is 150 ppm or less,
In addition, silicon nitride ceramics characterized by having a chlorine content of 250 ppm or less. 3. The silicon nitride ceramic according to claim 1, wherein the crystallized grain boundary phase is mainly composed of a melilite phase.